1. Field of the Invention
The present invention relates to a method for efficiently producing toner for developing an image of electrostatic charge which is superior in various properties such as chargeability and is suitable for use in the image formation in such application as electrophotography, toner for developing an image of electrostatic charge which is produced by the method, and a developer for an image of electrostatic charge and a method for forming an image by utilizing the toner for developing an image of electrostatic charge.
2. Description of the Related Art
A method in which image information is made visible via an image of electrostatic charge as in electrophotography and the like is widely used currently in various fields. Electrophotography consists of forming an image of electrostatic charge on a photoreceptor through charging and exposure steps, developing the image of electrostatic charge with a developer containing toner particles, and making the developed image visible via transfer and fixation steps.
By the way, there are two types of known developer, a two-component developer which comprises toner particles and carrier particles, and a one-component developer which comprises either magnetic toner particles or non-magnetic toner particles. These toner particles are usually prepared by a blending/pulverizing process. The blending/pulverizing process comprises melting and blending a thermoplastic resin with a pigment, a charge inhibitor and a release agent such as a wax, pulverizing the resultant product after cooling, and classifying the pulverized product by size to obtain the desired toner. If necessary, for the purpose of improving such properties as fluidity and cleanability of the thus prepared toner particles, inorganic and/or organic particles are added to the surface to the toner particles.
Usually, the shapes of the toner particles prepared in the above-mentioned blending/pulverizing process are irregular and the surface compositions of the toner particles are not uniform. The shapes and surface compositions of the toner particles vary slightly depending on the pulverizability of the materials and the pulverizing conditions. However, it is difficult to control the shapes and surface compositions of the toner particles within a desired range. Furthermore, if the materials of the toner particles are particularly easy to pulverize and the toner particles are prepared by the blending/pulverizing process, the toner particles are often pulverized further in a developing device by mechanical forces such as a shearing force causing the shapes of the toner particles to change. Accordingly, the problems to be encountered in the case of the two-component developer are accelerated degradation of the chargeability of the developer due to tenacious adhesion of the fine toner particles to the carrier surface, while the problems to be encountered in the case of the one-component developer are the broadening of the particle size distribution accompanied by the dissipation of the fine toner particles and the degradation of image due to diminished developing performance of the toner caused by the change of the shapes of the toner.
Further, if the shapes of the toner particles are irregular, a sufficient level of fluidity cannot be obtained even if a fluidity improving agent is added. Consequently, the problems are that the fluidity decreases with time because a mechanical force such as a shearing force causes the particles of the fluidity improving agent to move to cavities of the toner particles. When the improving agent becomes embedded in such cavities, performances such as developability, transferability and cleanability are reduced. In addition, if such toner particles are recovered by a cleaning treatment are used after being recycled to the developing device, the quality of the obtained image will be inferior. If the amount of the fluidity improving agent is increased in order to prevent the above-mentioned problems, new problems such as generation of black spots on the photoreceptor and the dissipation of the toner particles arise.
Meanwhile, if the toner particles contain a surface lubricant such as a wax, the release agent may be exposed to the surface of the toner particles depending on the combination of the release agent and a thermoplastic resin. In particular, if the toner particles utilize a combination of a resin whose elasticity is created by a polymeric component and which is somewhat difficult to pulverize and a brittle wax such as polyethylene, a significant proportion of the polyethylene is exposed to the surface of the toner particles. Although these toner particles are advantageous in terms of release in the fixing process and cleaning the photoreceptor of the untransferred toner, a mechanical force such as a shearing force inside the developing device causes the polyethylene to separate from the toner particles and to migrate to such members as developing rolls, a photoreceptor and carriers. Consequently, because of the contamination of these members, the reliability of the developer drops.
In order to overcome the above-mentioned problems, Japanese Patent Application Laid-Open (JP-A) Nos. 63-282,752 and 6-250,439 disclose an emulsion polymerization/flocculation process as a method for producing toner particles whose shapes and surface compositions are controlled. The emulsion polymerization/flocculation process comprises the steps of preparing a resin dispersion liquid by an emulsion polymerization on the one hand, preparing a colorant dispersion comprising a solvent and a colorant dispersed therein on the other hand, blending the two dispersions to prepare flocculated particles having a particle size corresponding to the toner particle diameter, and then heating the blend to fuse the particles and to obtain toner particles. According to the emulsion polymerization/flocculation process, it is possible to control the shapes of the toner particles at will from an irregular shape to a sphere by selecting the heating temperatures. A method of manufacturing toner by flocculating fine resin particles is outlined in Japanese Patent Application Laid-Open (JP-A) No. 6-282099. In this application, resins with high softening points and resins with low softening points are used.
In the emulsion polymerization/flocculation process, however, the problem is that it is difficult to control intentionally the structure and composition of the surface of the toner particles, because the composition in the region ranging from toner particle interior to the particle surface is made uniform by the fusion of the flocculated particles in a uniformly blended state. If the flocculated particles contain a release agent, the surface of the toner particles after fusion bears the release agent, which may lead to a filming phenomenon and the embedding of a fluidity improving agent employed as an external additive into the interior of the toner particles.
In an electrophotographic process, in order to maintain and exhibit the performance of toner in a stable manner, it is necessary to inhibit the exposure of the lubricant to the surface of toner particles, to increase the surface hardness of the toner particles and to increase the smoothness of the surface of the toner particles. Despite the possible problems arising from release agent exposure to the surface of the toner particles, it is preferable that the lubricant be present in the vicinity of the surface of the toner particles if the toner performance at the time of fixation is taken into consideration.
Recently, because of a mounting demand for high-quality images, the diameter of the toner particles is remarkably reduced in order to realize a high-precision image particularly in the formation of a color image. However, if the conventional particle size distribution of toner is maintained unchanged, it is difficult to achieve high quality images and high reliability simultaneously by the mere reduction in particle size of toner, because serious problems such as contamination of the carriers and photoreceptor as well as dissipation of the toner particles are caused by toner particles existing in a region of finer particles of the particle size distribution. In order to achieve high quality images and high reliability simultaneously, it is necessary to sharpen the particle size distribution of the toner particles and reduce the particle size.
Accordingly, the present invention intends to overcome the problems of prior art and to achieve the following objectives.
In the present invention, the structure and composition in the region ranging from the surface to the interior of the toner particle are controlled to achieve the following objectives:
1. To provide toner for developing an image of electrostatic charge which is superior in various properties such as chargeability, developability, transferability, fixability and cleanability and particularly in chargeability as well as a developer utilizing the toner for developing an image of electrostatic charge; PA1 2. To provide toner for developing an image of electrostatic charge which is capable of maintaining and exhibiting the above-mentioned properties and particularly the chargeability without being influenced by environmental conditions and which has a high reliability as well as a developer utilizing the toner for developing an image of electrostatic charge; PA1 3. To provide toner for developing an image of electrostatic charge suitable to a two-component developer for an image of electrostatic charge which consumes a small amount of the toner and yet has a long life; PA1 4. To provide an easy and simple method for producing toner for developing an image of electrostatic charge which is superior in the above-mentioned properties; PA1 5. To provide an easy and simple method for forming a full-color image having a high-quality and a high reliability; PA1 6. To provide a method for forming an image which ensures a high quality of image in a system having no cleaning mechanism, namely, a cleaner-less system; and PA1 7. To provide a method for forming an image which is highly adapted to a toner-recycle system reusing the toner recovered from a cleaner and which ensures a high quality of image.